In fact, different classes of hydrogels are known depending on the chemical or physical structure of the latter. An often used classification, especially in the biomedical field, distinguishes between neutral or non-ionic hydrogels (of the cellulose type, for example), ionic hydrogels (of the silicate type, for example) and interpenetrated polymeric networks (IPN). These known hydrogels generally have water-swollen networks (crosslinked structures) of hydrophilic polymers or copolymers. These networks are three-dimensional and the crosslinkings are formed by covalent or ionic bonds Frequently weaker bonds of the Van der Waals type, or hydrogen bond, can serve as crosslinks thus forming the swollen networks which behave as hydrogels.
Finally, semi-crystalline and un-crosslinked hydrophilic polymers can form hydrogels by swelling then the crystallites act as physical crosslinks, it being understood that they do not dissolve in water, unlike amorphous constituents.